In the Semiconductor industry, Rapid Thermal Processing (RTP) is used to influence diffusion of dopants into silicon and other substrates. Such processing is used to produce doped regions having appropriate thickness and conductive characteristics. Since reaction rates and diffusion rates are a function of temperature, control of temperature affects the performance of devices manufactured as integrated circuits. If the temperature on one region of the device differs from that of another region of the device, the behavior of circuits in those differing regions can have different performance characteristics.
Control of performance characteristics is relatively more important for small featured devices, such as those having 0.13 μm features or less, than for larger featured devices. For example, temperature influences activation of source/drain implants which affects transistor leakage, device size, and ultimately, clock speed. Depending on the specific process, each 1° C. of temperature variation across the device can change the L-effective of the device by as much as 1 nm or more.
A typical Rapid Thermal Anneal (RTA) may raise the temperature of a wafer to 950–1200° C. in less than a minute. This process is often performed with Tungsten Halogen lamps, which direct light onto the surface of the wafer. The surface material then absorbs the energy to varying extents depending on the absorptivity of the surface material. Higher absorptivity materials absorb more energy and thus heat faster than lower absorptivity materials. Because of the varied surface materials on a semiconductor device, the temperature at a location within the wafer can be dependent on the absorptivity of nearby materials.
The design of very large-scale integrated circuits often utilizes automated algorithms for presenting a design layout. Often these algorithms determine positioning of transistors about an integrated circuit based on timing considerations assuming a common transistor performance. After transistor placement, remaining space can be filled with relatively large capacitors, as needed. During manufacture, these areas with large capacitors typically have large dense areas of polysilicon while transistor dense areas have less dense arrangements of polysilicon. Polysilicon is a low absorptivity material. Therefore, regions with high polysilicon surface density absorb less energy and heat slower than the lower density polysilicon areas. For this reason, maintaining minimal temperature variance across the circuit is difficult during rapid thermal processes (RTP). Regions about the capacitors absorb less energy and thus have lower temperatures than other regions of the integrated circuit. The problem of having varied temperatures across a circuit is especially troublesome for Semiconductor-On-Insulator circuits having buried oxide (BOX) layers that are also poor conductors. With poor thermal conduction under the capacitors due to the BOX layers, thermal energy in one region of the wafer is insulated from spreading efficiently to other regions and thereby reducing the affects of varied surface absorptivity.
The rapid nature of the temperature increase during RTA prevents traditional temperature control methods such as slower temperature ramping from being employed. Slower temperature ramping more closely matches thermal conduction time and length scales. However, slow temperature ramping fails to produce the desired device characteristics and increases manufacturing time.
As such, many typical wafer designs suffer from deficiencies in uniform processing temperatures and thus uniform performance. Other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present disclosure as described herein.